The present invention relates to improvements in the hard-facing of metal parts by forming borides thereon through welding-type melting of surface-borided filler members, and, in one particular aspect, to novel and improved hard-facing and the like wherein torch-melted filler rods or wires are of relatively ductile metal into which boron has been diffused to leave relatively brittle boride about a relatively ductile core.
It has been well known that conversion of surfaces of a body metal, such as iron, to borides (for example, FeB, Fe.sub.2 B) can result in the formation of exteriors which are relatively hard and offer improved resistances to wear and to certain corrosives. Such conversions may result from reactions with the body metal which leave the shaping and finish essentially unchanged, but it has also been well known to harden and/or restore worn members via added-on layers or cladding of such materials. Using the latter techniques, it has been possible to hard-face worn parts conveniently at job sites, and to take advantage of toughness and economy of common alloys in fabricating bulky items which need be hard only along wear surfaces. Hard-facing involves a build-up of the wear-resistant material through melting and filling with the aid of a generally conventional welding torch and a filler rod or wire of the hardening material.
The embrittling effects which follow along with the hardening developed by boron in various metals has led to powder-metallurgy practices, wherein boride particles have been bonded together to form larger masses, such as are described in U.S. Pat. Nos. 1,913,373 and 1,913,100 and 1,968,067. Alloys with boride filaments to enhance hardness are also known, as is described in U.S. Pat. No. 3,507,632. Brazing alloy paste including boron is discussed in U.S. Pat. No. 3,024,109, where the effects of boron upon melting temperatures are recognized, and protective coatings are described in U.S. Pat. No. 3,666,436 where the particulated mixture used to produce the coatings include borides. Diffusion-bonding of members, with the aid of boronized layers, has been discussed in a number of U.S. Pat. Nos. including 3,197,858 and 3,339,269 and 3,678,570 and 3,753,794; however, the objectives and techniques there will be seen to be different from those of the hard-facing with which the present invention is concerned.
Although one may attempt to use powdered borides, in compacted forms, as fillers in welding-type hard-facing operations, there is a tendency for such particulate materials to separate and result in facings which are non-uniform; in addition, the borides which have been compacted or cast into the form of slender filler rods tend to be difficult to handle because of their brittleness, and that characteristic prevents their being used in substantial lengths or coiled in spools for automatic or continuous feeding.